1. Field of the Invention
The present invention relates to a heat transfer pipe for use in freezing, air conditioning, etc. as well as a method of making the same.
2. Description of the Prior Art
Heretofore, as heat transfer pipes for use in heat pump air conditioners or the like, there have mainly been used internally grooved pipes from the standpoint of attaining a high efficiency and saving energy. In such internally grooved pipes, fine triangular or trapezoidal grooves are formed straight or spirally in the inner surfaces of the pipes, as shown in Japanese Patent Laid Open No. 37059/79 and U.S. Pat. Nos. 4,044,797 and 4,373,366. Particularly, in a spirally grooved pipe, there is attained a superior boiling heat transfer characteristic because a corner portion of the internal groove forms a boiling nucleus, in addition to an improvement in the stirring action for internal fluid and an increase in the heat transfer rate due to increase of the internal surface area. Further, there have been developed a crosswise grooved pipe having a superior heat transfer characteristic in terms of evaporation and boiling as well as a heat transfer pipe having an improved vaporization heat characteristic and having a tunnel-like cavity formed in the inner wall surface thereof.
In a heat pump air conditioner, however, when the temperature of the outside air falls in winter for example, it is impossible to attain a sufficient evaporation of refrigerant because an evaporator is provided outdoors, thus often resulting in lowering of the heating temperature. This is due to deterioration of the vaporization heat characteristic of the heat transfer pipe used in the evaporator and is conspicuous particularly when the temperature is low.
As measures to avoid such inconvenience, improvements have been made with respect to the number of groove threads, lead angle, crossing and shape in conventional internally grooved pipes. But since there is a limit in such improvements, it is impossible to expect improving the heat transfer characteristic without deterioration of the con-ensation characteristic.
Further, if a heat transfer pipe having the above tunnel-like cavity is used, the number of sharp projections decreases, so the condensation characteristic deteriorates.
An outdoor machine of the heat pump air conditioner functions as a condenser during the summer season, so the deterioration of the condensation characteristic causes deficiency in the cooling capacity during the summer season.